Digital-to-analog conversion apparatus and method having signal calibration mechanism

ABSTRACT

The present invention discloses a DAC method having signal calibration mechanism is provided. Operation states of current sources are controlled to generate an output analog signal by a DAC circuit according to a codeword of an input digital signal. An echo signal is generated by an echo transmission circuit according to the output analog signal. The codeword is mapped to generate an offset signal by a calibration circuit according to a codeword offset mapping table. The offset signal is processed to generate an echo-canceling signal by an echo-canceling circuit. By a calibration parameter calculation circuit, offset amounts are generated according to a difference between the echo signal and the echo-canceling signal, the offset amounts are grouped to perform statistic operation according to the operation states and current offset values are calculated according to calculation among groups and converted to codeword offset values to update the codeword offset mapping table.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to a digital-to-analog conversion (DAC)apparatus and a digital-to-analog conversion method having signalcalibration mechanism.

2. Description of Related Art

A DAC circuit is an important circuit component that converts a signalfrom a digital form to an analog form. Based on different digital codes,the DAC circuit can multiply the digital codes by a correspondingconversion gain value to generate analog signals with differentintensities.

However, for a current-output type of DAC circuit, a manufacturingprocess offset results in non-ideal offsets of amplitude and shape ofthe outputted analog signal. The offset of amplitude especially affectsthe system performance seriously.

SUMMARY OF THE INVENTION

In consideration of the problem of the prior art, an object of thepresent invention is to supply a digital-to-analog conversion apparatusand a digital-to-analog conversion method having signal calibrationmechanism.

The present invention discloses a digital-to-analog conversion (DAC)apparatus having signal calibration mechanism that includes a DACcircuit, an echo transmission circuit, a calibration circuit, anecho-canceling circuit and a calibration parameter calculating circuit.The DAC circuit includes a plurality of current sources each having acurrent offset, and is configured to control an operation state of eachof the current sources to be one of a first current output state and asecond current output state according to an input digital codewordincluded by an input digital signal, so as to generate an output analogsignal according to a total current of the current sources, wherein eachof the current sources outputs a current with a same current amount butopposite signs respectively under the first current output state and thesecond current output state. The echo transmission circuit processes theoutput analog signal to generate an echo signal. The calibration circuitreceives the input digital signal and perform mapping from a codewordoffset mapping table according to the input digital codeword to generatean offset, wherein the codeword offset mapping table includes aone-to-one correspondence relation between a plurality of codewords anda plurality of codeword offsets. The echo-canceling circuit processesthe offset according to a group of echo-canceling coefficients togenerate an echo-canceling signal. The calibration parameter calculatingcircuit is configured to generate an offset amount according to adifference between the echo signal and the echo-canceling signal, tocategorize the offset amount corresponding to the different inputdigital codeword into a plurality of groups according to the operationstate of each of the current sources to perform statistic operation, toset each of the current sources as a target current source to performcalculation among the groups such that the current offset of each of thecurrent sources besides the target current source cancels out tocalculate the current offset of the target current source and to convertthe current offset of the current sources to the codeword offsets toupdate the codeword offsets in the codeword offset mapping table.

The present invention also discloses a digital-to-analog conversionmethod having signal calibration mechanism that includes the stepsoutlined below. An operation state of each of a plurality of currentsources is controlled to be one of a first current output state and asecond current output state, by a DAC circuit comprising the currentsources each having a current offset, according to an input digitalcodeword included by an input digital signal, so as to generate anoutput analog signal according to a total current of the currentsources, wherein each of the current sources outputs a current with asame current amount but opposite signs respectively under the firstcurrent output state and the second current output state. The outputanalog signal is processed to generate an echo signal by an echotransmission circuit. The input digital signal is received and mappingis performed from a codeword offset mapping table according to the inputdigital codeword by a calibration circuit to generate an offset, whereinthe codeword offset mapping table includes a one-to-one correspondencerelation between a plurality of codewords and a plurality of codewordoffsets. The offset is processed according to a group of echo-cancelingcoefficients by an echo-canceling circuit to generate an echo-cancelingsignal. An offset amount is generated according to a difference betweenthe echo signal and the echo-canceling signal by a calibration parametercalculating circuit. The offset amount corresponding to the differentinput digital codeword is categorized into a plurality of groupsaccording to the operation state of each of the current sources toperform statistic operation by the calibration parameter calculatingcircuit. Each of the current sources is set as a target current sourceto perform calculation among the groups by the calibration parametercalculating circuit such that the current offset of each of the currentsources besides the target current source cancels out to calculate thecurrent offset of the target current source. The current offset of thecurrent sources is converted to the codeword offsets to update thecodeword offsets in the codeword offset mapping table by the calibrationparameter calculating circuit.

These and other objectives of the present invention will no doubt becomeobvious to those of ordinary skill in the art after reading thefollowing detailed description of the preferred embodiments that areillustrated in the various figures and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a block diagram of a digital-to-analog conversionapparatus having signal calibration mechanism according to an embodimentof the present invention.

FIG. 2 illustrates a circuit diagram of the DAC circuit according to anembodiment of the present invention.

FIG. 3 illustrates a diagram of the groups that categorize the offsetamount according to the operation state of each of the current sourcesaccording to an embodiment of the present invention.

FIG. 4 illustrates a more detailed diagram of the groups according to anembodiment of the present invention.

FIG. 5 illustrates a flow chart of a digital-to-analog conversion methodhaving signal calibration mechanism according to an embodiment of thepresent invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

An aspect of the present invention is to provide a digital-to-analogconversion apparatus and a digital-to-analog conversion method havingsignal calibration mechanism to perform calibration on the offset ofeach of different codewords to avoid the error caused by the offset ofthe output signal due to the offset of the codewords.

Reference is now made to FIG. 1. FIG. 1 illustrates a block diagram of adigital-to-analog conversion apparatus 100 having signal calibrationmechanism according to an embodiment of the present invention. Thedigital-to-analog conversion apparatus 100 includes a DAC circuit 110,an echo transmission circuit 120, a calibration circuit 130, anecho-canceling circuit 140 (abbreviated as ECC in FIG. 1), an errorcalculating circuit 150, an inverted error calculating circuit 160(abbreviated as ICC in FIG. 1), a calibration parameter calculatingcircuit 170 (abbreviated as CPC in FIG. 1) and an auxiliary DAC circuit180.

The DAC circuit 110 receives an input digital signal IS including aninput digital codeword from a signal source SS to performdigital-to-analog conversion to generate an output analog signal OAS.The signal source SS can be such as, but not limited to a transmissioncircuit (TX) of a communication system.

Reference is now made to FIG. 2 at the same time. FIG. 2 illustrates acircuit diagram of the DAC circuit 110 according to an embodiment of thepresent invention.

The DAC circuit 110 includes a plurality of current sources CA˜CS. Theinput digital codeword of the input digital signal IS can control theoperation state of each of the current sources CA˜CS to be one of afirst current output state and a second current output state, so as togenerate the output analog signal OAS according to a total current ofthe current sources CA˜CS. Each of the current sources CA˜CS outputs acurrent with the same current amount but opposite signs respectivelyunder the first current output state and the second current output state

In an embodiment, the input digital codeword of the input digital signalIS includes a plurality of thermometer codes TC to control a pluralityof thermometer-control current sources CA˜CO of the current sources.Each of the current sources CA˜CO outputs one unit of current. In anembodiment, the thermometer codes TC controls the switching circuit 200corresponding to the current sources CA˜CO such that the current sourcesCA˜CO are electrically coupled to different current output paths throughthe switching circuit 200, e.g., one of the solid-line paths and thedotted-line paths illustrated in FIG. 2, so as to be operated in one ofthe first current output state and the second current output state.

The thermometer codes TC having a number of A have 2A code combinationsthat are able to control the thermometer-control current sources havinga number of 2A-1. In the present embodiment, A is 4. As a result, 4thermometer codes TC control 15 thermometer-control current sourcesCA˜CO according to 16 combinations.

In operation, when the thermometer codes TC switch from the combinationsof (0000), (0001), . . . t o (1111) in order, the current sources CA˜COswitches the operation state thereof in order like a thermometer. As aresult, the operation of the current sources CA˜CO starts from all beingin the second current output state, through gradually switching thecurrent sources CA to the current sources CN from the second currentoutput state to the first current output state, to all being in thefirst current output state.

In an embodiment, the input digital codeword of the input digital signalIS includes a plurality of binary codes BC to control a plurality ofbinary-control current sources CP˜CS of the current sources. The currentsources CP˜CS respectively output ½ unit, ¼ unit, ⅛ unit and 1/16 unitof current. In an embodiment, the binary codes BC controls the switchingcircuit 210 corresponding to the current sources CP˜CS such that thecurrent sources CP˜CS are electrically coupled to different currentoutput paths through the switching circuit 210, such as one of thesolid-line paths and the dotted-line paths illustrated in FIG. 2, so asto be operated in one of the first current output state and the secondcurrent output state.

The binary codes BC having a number of B have 2^(B) combinations thatare able to control the binary-control current sources having a numberof B. In the present embodiment, B is 4. As a result, 4 binary codes BCcontrol 4 binary-control current sources CP˜CS according to 16combinations.

In operation, when the binary codes BC switch from the combinations of(0000), (0001), . . . to (1111) in order, the current source CP operatesin the second current output state and the first current output staterespectively when the highest bit is 0 and 1. The current source CQoperates in the second current output state and the first current outputstate respectively when the second highest bit is 0 and 1. The currentsource CR operates in the second current output state and the firstcurrent output state respectively when the second lowest bit is 0 and 1.The current source CS operates in the second current output state andthe first current output state respectively when the lowest bit is 0 and1.

As a result, the DAC circuit 110 can generate an output analog signalOAS according to the total current of the current sources CA˜CS by usingthe thermometer codes TC to control the switching circuit 200 and usingthe binary codes BC to control the switching circuit 210.

Due to the manufacturing process offset, each of the current sourcesCA˜CS has a current offset. The thermometer-control current sourceshaving the number of 2^(A)-1 generate 2^(A)-1 combinations of currentoffsets. The binary-control current sources having the number of Bgenerate B combinations of current offsets. All the current sourcesgenerate 2^(A)-1+B combinations of current offsets.

According to different input digital codewords, these current offsetsresult in different codeword offsets. In the numerical example describedabove, 15 thermometer-control current sources and 4 binary-controlcurrent sources result in 19 combinations of current offsets. The 19current sources are controlled by 256 combinations of the input digitalcodeword, wherein the input digital codeword has 8 bits (4 bits ofthermometer codes and 4 bits of binary codes). As a result, 19combinations of current offsets result in 256 combinations of codewordoffsets.

The echo transmission circuit 120 processes the output analog signal OASto generate an echo signal ES.

In an embodiment, the echo transmission circuit 120 includes an echoresponse circuit 190 and analog-to-digital conversion (ADC) circuit 195.The echo response circuit 190 performs an echo response processing onthe output analog signal OAS and the analog-to-digital conversioncircuit 195 performs analog-to-digital conversion subsequently togenerate an echo signal ES. In an embodiment, the echo transmissioncircuit 120 may selectively include such as, but not limited to alow-pass filter or other digital signal processing circuit (notillustrated in the figure) to perform further digital processing on theecho signal ES.

The calibration circuit 130 receives the input digital signal IS andperforms mapping from a codeword offset mapping table TB according tothe input digital codeword of the input digital signal IS to generate anoffset DS. The codeword offset mapping table TB includes a one-to-onecorrespondence relation between a plurality of codewords and a pluralityof codeword offsets, and the input digital codeword corresponds to oneof these codewords. Take the 8-bit form of codeword as an example, thecodeword offset mapping table TB includes 256 correspondence relationssuch that each of the 256 codewords corresponds to one of the 256offsets. In an embodiment, in an initial state, each of all the offsetsthat the codewords correspond to is preset to be 0.

The echo-canceling circuit 140 processes the offset DS according to agroup of echo-canceling coefficients CEC to generate an echo-cancelingsignal ECS. It is appreciated that in an embodiment, the echo-cancelingcircuit 140 can be shared with the receiving circuit (RX) of thecommunication system to cancel the echo signal fed to the receivingcircuit from the transmission circuit.

The calibration parameter calculating circuit 170 generates an offsetamount DA according to a difference between the echo signal ES and theecho-canceling signal ECS. The echo signal ES and the echo-cancelingsignal ECS may be selectively processed by the error calculating circuit150 and the inverted error calculating circuit 160 first and processedby the calibration parameter calculating circuit 170 subsequently.

The error calculating circuit 150 performs subtraction between the echosignal ES and the echo-canceling signal ECS to generate an error signalDIS. In an initial state, the echo-canceling circuit 140 performstraining process on the echo-canceling coefficients CEC according to theerror signal DIS such that the echo-canceling coefficients CEC convergesand applies response processing on the offset DS according to theconverged echo-canceling coefficients CEC. Such a response processing isidentical to the response of a path including the DAC circuit 110 andthe echo response circuit 190 that transmits the input digital signal ISfrom the signal source SS.

Subsequently, the inverted error calculating circuit 160 performsone-dimensional inversion on the echo-canceling coefficients CEC andrespectively performs multiplication thereon by the value of the errorsignal DIS to generate an inverted error value FD. The calibrationparameter calculating circuit 170 can make the inverted error value FDserve as the offset amount DA that the input digital codewordcorresponds to according to a path delay amount DL of the echo-cancelingcircuit 140 and the inverted error calculating circuit 160.

It is appreciated that the generation of the offset amount describedabove is merely an example. In other embodiments, the calibrationparameter calculating circuit 170 may generate the offset amount DA byusing other methods.

Further, the calibration parameter calculating circuit 170 categorizesthe offset amount DA corresponding to the different input digitalcodeword into a plurality of groups according to the operation state ofeach of the current sources CA˜CO to perform statistic operation. Morespecifically, the calibration parameter calculating circuit 170categorizes the offset amount DA corresponding to the different inputdigital codeword into one of the groups that corresponds to acombination of the thermometer codes, and generates an average value ofthe offset amount DA of each of the groups. The groups are arranged inan order such that for each two of the neighboring groups, the operationstate of one of the thermometer-control current sources is at the firstcurrent output state and the second current output state respectively.

Reference is now made to FIG. 3. FIG. 3 illustrates a diagram of thegroups G01˜G16 that categorize the offset amount DA according to theoperation state of each of the current sources CA˜CO according to anembodiment of the present invention.

In an embodiment, the current offsets of the current sources CAC are ΔA˜ΔO. When the current sources CA˜CO are under the first current outputstate, the current offsets are +ΔA˜+ΔO. When the current sources CA˜COare under the second current output state, the current offsets are−ΔA˜−ΔO.

The group G01 corresponds to the thermometer code (0000) that makes thecurrent sources CA˜CO all operate under the second current output statewhile the binary codes can be any values (0000˜1111). As a result, 16input digital codewords (00000000˜00001111) belong to the group G01.After a certain amount of offset amounts DA categorized into the groupG01 are averaged, the current offset of the current sources CP˜CS (thebinary-control current sources) cancel out each other such that theaverage value approximates a sum of the current offset of the currentsources CA˜CO, which is −ΔA−ΔB−ΔC−. . . −ΔO.

Identically, the average value of each of the group G02 to the group G16that corresponds the each of the thermometer codes (0001˜1111) can beobtained by using the derivative method described above. The detail isnot described herein.

The calibration parameter calculating circuit 170 sets each of thecurrent sources CA˜CO as a target current source to perform calculationamong the groups G01˜G16 such that the current offset of each of thecurrent sources besides the target current source cancels out tocalculate the current offset of the target current source.

More specifically, the calibration parameter calculating circuit 170generates one of the current offsets of the thermometer-control currentsources according to a difference of the average values of each two ofthe neighboring groups. For example, the difference of the averagevalues between the group G02 and the group G01 can be expressed by thefollowing equation:

(+ΔA−ΔB−ΔC−. . . −ΔO)−(−ΔA−ΔB−ΔC−. . . −ΔO)=+2ΔA

As a result, the calibration parameter calculating circuit 170 cangenerate the current offset ΔA of the current source CA by using thecalculation described above.

Identically, the calibration parameter calculating circuit 170 cangenerate the current offsets ΔB˜ΔO each corresponding to one of thecurrent sources CB˜CO according to the differences of the average valuesbetween the groups G03 and G02, between the groups G04 and G03, . . .and between the groups G16 and G15.

In an embodiment, the calibration parameter calculating circuit 170 setsthe average value of each of the group G01 and the group G16 to be 0 asanchor points, and generates the current offset of one of thethermometer-control current sources according to a difference betweenthe average values of each two of the neighboring groups. Theinteraction within the system can thus be avoided.

Reference is now made to FIG. 4. FIG. 4 illustrates a more detaileddiagram of the group G01 and the group G16 according to an embodiment ofthe present invention.

Besides the current offsets of the thermometer-control current sources,the offset amounts DA categorized into each of the groups also includethe 16 combinations of the current offsets of the binary-control currentsource.

Take group G01 and the group G16 as an example, the current offsets ofthe current sources CP˜CS that are the binary-control current sourcesare ΔP˜+ΔS. When the current sources CP˜CS are under the first currentoutput state, the current offsets are +ΔP˜+ΔS. When the current sourcesCP˜CS are under the second current output state, the current offsets are−ΔP˜−ΔS. As a result, according to the operation state of the each ofthe current sources CP˜CS, the current offset of each of the groups has16 combination illustrated in FIG. 4. In FIG. 4, the 16 combinationscorresponding to the group G01 are labeled as P01˜P16 respectively. The16 combinations corresponding to the group G16 are labeled as N01˜N16respectively.

The calibration parameter calculating circuit 170 selects a first groupand a second group from the groups G01˜G16, wherein the thermometercodes corresponding to the first group and the second group make theoperation state of each of the thermometer-control current sourcescompletely opposite. Further, the calibration parameter calculatingcircuit 170 averages the offset amount DA of each of the binary-controlcurrent sources that corresponds to the first current output state inthe first group and the second group to generate the current offset ofone of the binary-control current sources.

Take the group G01 and the group G16 as an example, the thermometer codecorresponding to the group G01 (0000) makes all the current sourcesCA˜CO operate under the second current output state. The thermometercode corresponding to the group G16 (1111) makes all the current sourcesCA˜CO operate under the first current output state.

Take the current source CP as an example, the calibration parametercalculating circuit 170 categorizes the offset amounts DA that make thecurrent source CP operate under the first current output state andperform average thereon. As illustrated in FIG. 4, the offset amounts DAthat make the current source CP operate under the first current outputstate corresponding to the combinations N09˜N16 and P09˜P16 and aremarked with white dots. After averaging these offset amounts DA, thecurrent offsets ΔQ˜ΔS in these combinations cancel out such that onlythe current offset ΔP of the current source CP remains.

The current offsets ΔQ˜ΔS can be generated by using the same method. Forexample, the current offset ΔQ can be generated by performing averagingon the combinations marked with black dots. The current offset ΔR can begenerated by performing averaging on the combinations marked with whitegrids. The current offset ΔS can be generated by performing averaging onthe combinations marked with black grids. The detail calculation is notdescribed herein. It is appreciated that the selection of the group G01and the group G16 is merely an example. In other embodiments, thecalibration parameter calculating circuit 170 may select other twogroups having the thermometer-control current sources operate incompletely opposite states.

The calibration parameter calculating circuit 170 converts the currentoffsets ΔA˜ΔS to the codeword offsets to update the codeword offsets inthe codeword offset mapping table TB in FIG. 1. In an embodiment, thecalibration parameter calculating circuit 170 can accumulate more piecesof the offset amounts DA within every certain time interval to furtherperform calculation of the current offsets so as to update the codewordoffset mapping table TB.

After the codeword offsets are updated to a stable condition, theauxiliary DAC circuit 180 receives the input digital signal IS andperforms mapping from the codeword offset mapping table TB according tothe input digital codeword to generate an offset calibration analogsignal CAS. The absolute value of the offset calibration analog signalCAS is equivalent to the absolute value of the offset that the inputdigital codeword corresponds to. The digital-to-analog conversionapparatus 100 may further includes an adding circuit 185 to add theoffset calibration analog signal CAS and the output analog signal OAS tocancel the offset that the input digital codeword corresponds to andgenerate an actual output analog signal AAS.

It is appreciated that the offset may be a positive value or a negativevalue. When the offset is a positive value, the offset calibrationanalog signal CAS is a negative value. When the offset is a negativevalue, the offset calibration analog signal CAS is a positive value.

As a result, the digital-to-analog conversion apparatus of the presentinvention performs calculation of the current offsets and converts thecurrent offsets to the offsets of the codewords to perform calibrationon the offsets of different codewords, in which the number of thecurrent offsets is less than the number of the codewords. Not only theoffset of the signal can be calibrated, the mass calculation amount thatis required to directly calculate the codeword offsets can be avoided.

Reference is now made to FIG. 5. FIG. 5 illustrates a flow chart of adigital-to-analog conversion method 500 having signal calibrationmechanism according to an embodiment of the present invention.

In addition to the apparatus described above, the present disclosurefurther provides the digital-to-analog conversion method 500 that can beused in such as, but not limited to, the digital-to-analog conversionapparatus 100 in FIG. 1. As illustrated in FIG. 5, an embodiment of thedigital-to-analog conversion method 500 includes the following steps.

In step S510, the operation state of each of the current sources CA˜CSis controlled to be one of the first current output state and the secondcurrent output state, by the DAC circuit 110 including the currentsources CA˜CS each having a current offset, according to an inputdigital codeword included by an input digital signal IS, so as togenerate the output analog signal OAS according to the total current ofthe current sources CA˜CS, wherein each of the current sources CA˜CSoutputs the current with the same current amount but opposite signsrespectively under the first current output state and the second currentoutput state.

In step S520, the output analog signal OAS is processed to generate theecho signal ES by the echo transmission circuit 120.

In step S530, the input digital signal IS is received and mapping isperformed from the codeword offset mapping table TB according to theinput digital codeword by the calibration circuit 130 to generate theoffset DS, wherein the codeword offset mapping table TB includes theone-to-one correspondence relation between the codewords and thecodeword offsets.

In step S540, the offset DS is processed according to the group ofecho-canceling coefficients CEC by the echo-canceling circuit 140 togenerate the echo-canceling signal ECS.

In step S550, the offset amount DA is generated according to thedifference between the echo signal ES and the echo-canceling signal ECSby the calibration parameter calculating circuit 170.

In step S560, the offset amount DA corresponding to the different inputdigital codeword is categorized into the groups according to theoperation state of each of the current sources CA˜CS to performstatistic operation by the calibration parameter calculating circuit170.

In step S570, each of the current sources CA˜CS is set as the targetcurrent source to perform calculation among the groups by thecalibration parameter calculating circuit 170 such that the currentoffset of each of the current sources CA˜CS besides the target currentsource cancels out to calculate the current offset of the target currentsource.

In step S580, the current offset of the current sources CA˜CS isconverted to the codeword offsets to update the codeword offsets in thecodeword offset mapping table TB by the calibration parametercalculating circuit 170.

It is appreciated that the embodiments described above are merely anexample In other embodiments, it should be appreciated that manymodifications and changes may be made by those of ordinary skill in theart without departing, from the spirit of the disclosure.

In summary, the present invention discloses the digital-to-analogconversion apparatus and the digital-to-analog conversion method havingsignal calibration mechanism that performs calculation of the currentoffsets and converts the current offsets to the offsets of the codewordsto perform calibration on the offsets of different codewords, in whichthe number of the current offsets is less than the number of thecodewords. Not only the offset of the signal can be calibrated, the masscalculation amount that is required to directly calculate the codewordoffsets can be avoided.

The aforementioned descriptions represent merely the preferredembodiments of the present invention, without any intention to limit thescope of the present invention thereto. Various equivalent changes,alterations, or modifications based on the claims of present inventionare all consequently viewed as being embraced by the scope of thepresent invention.

What is claimed is:
 1. A digital-to-analog conversion (DAC) apparatushaving signal calibration mechanism, comprising: a DAC circuitcomprising a plurality of current sources each having a current offset,and configured to control an operation state of each of the currentsources to be one of a first current output state and a second currentoutput state according to an input digital codeword comprised by aninput digital signal, so as to generate an output analog signalaccording to a total current of the current sources, wherein each of thecurrent sources outputs a current with a same current amount butopposite signs respectively under the first current output state and thesecond current output state; an echo transmission circuit to process theoutput analog signal to generate an echo signal; a calibration circuitto receive the input digital signal and perform mapping from a codewordoffset mapping table according to the input digital codeword to generatean offset, wherein the codeword offset mapping table comprises aone-to-one correspondence relation between a plurality of codewords anda plurality of codeword offsets; an echo-canceling circuit to processthe offset according to a group of echo-canceling coefficients togenerate an echo-canceling signal; and a calibration parametercalculating circuit configured to: generate an offset amount accordingto a difference between the echo signal and the echo-canceling signal;categorize the offset amount corresponding to the different inputdigital codeword into a plurality of groups according to the operationstate of each of the current sources to perform statistic operation; seteach of the current sources as a target current source to performcalculation among the groups such that the current offset of each of thecurrent sources besides the target current source cancels out tocalculate the current offset of the target current source; and convertthe current offset of the current sources to the codeword offsets toupdate the codeword offsets in the codeword offset mapping table.
 2. Thedigital-to-analog conversion apparatus of claim 1, further comprising anauxiliary DAC circuit to receive the input digital signal and performmapping from the codeword offset mapping table according to the inputdigital codeword to generate an offset calibration analog signal, suchthat the offset calibration analog signal and the output analog signalare added to cancel the codeword offsets that the input digital codewordcorresponds to output an actual output analog signal.
 3. Thedigital-to-analog conversion apparatus in claim 1, wherein the inputdigital codeword comprises a plurality of thermometer codes to control aplurality of thermometer-control current sources of the current sources,the calibration parameter calculating circuit is further configured to:categorize the offset amount corresponding to the different inputdigital codeword into one of the groups that corresponds to acombination of the thermometer codes and generate an average value ofthe offset amount of each of the groups, wherein the groups are arrangedin an order such that for each two of the neighboring groups, theoperation state of one of the thermometer-control current sources is atthe first current output state and the second current output staterespectively; and generate the current offset of one of thethermometer-control current sources according to a difference of theaverage value between each two of the neighboring groups.
 4. Thedigital-to-analog conversion apparatus in claim 3, wherein the inputdigital codeword further comprises a plurality of binary codes tocontrol a plurality of binary-control current sources of the currentsources, the calibration parameter calculating circuit is furtherconfigured to: select a first group and a second group from the groups,wherein the thermometer codes corresponding to the first group and thesecond group make the operation state of each of the thermometer-controlcurrent sources completely opposite; and average the offset amount ofeach of the binary-control current sources that corresponds to the firstcurrent output state in the first group and the second group to generatethe current offset of one of the binary-control current sources.
 5. Thedigital-to-analog conversion apparatus in claim 4, wherein a number ofthe thermometer codes is A, the number of the thermometer-controlcurrent sources is 2^(A)-1, the number of the groups is 2^(A), thenumber of the binary codes is B, the number of the binary-controlcurrent sources is B, and the number of the combination of the offsetamount comprised in each of the groups is 2 ^(B), and the number of thecurrent offsets is 2^(A)-1+B.
 6. The digital-to-analog conversionapparatus in claim 4, wherein the calibration parameter calculatingcircuit sets the average value of each of the first group and the secondgroup to be 0, and generates the current offset of one of thethermometer-control current sources according to a difference betweenthe average values of each two of the neighboring groups.
 7. Adigital-to-analog conversion method having signal calibration mechanism,comprising: controlling an operation state of each of a plurality ofcurrent sources to be one of a first current output state and a secondcurrent output state, by a digital-to-analog conversion (DAC) circuitcomprising the current sources each having a current offset, accordingto an input digital codeword comprised by an input digital signal, so asto generate an output analog signal according to a total current of thecurrent sources, wherein each of the current sources outputs a currentwith a same current amount but opposite signs respectively under thefirst current output state and the second current output state;processing the output analog signal to generate an echo signal by anecho transmission circuit; receiving the input digital signal andperforming mapping from a codeword offset mapping table according to theinput digital codeword by a calibration circuit to generate an offset,wherein the codeword offset mapping table comprises a one-to-onecorrespondence relation between a plurality of codewords and a pluralityof codeword offsets; processing the offset according to a group ofecho-canceling coefficients by an echo-canceling circuit to generate anecho-canceling signal; generating an offset amount according to adifference between the echo signal and the echo-canceling signal by acalibration parameter calculating circuit; categorizing the offsetamount corresponding to the different input digital codeword into aplurality of groups according to the operation state of each of thecurrent sources to perform statistic operation by the calibrationparameter calculating circuit; setting each of the current sources as atarget current source to perform calculation among the groups by thecalibration parameter calculating circuit such that the current offsetof each of the current sources besides the target current source cancelsout to calculate the current offset of the target current source; andconverting the current offset of the current sources to the codewordoffsets to update the codeword offsets in the codeword offset mappingtable by the calibration parameter calculating circuit.
 8. Thedigital-to-analog conversion method in claim 7, further comprising:receiving the input digital signal and performing mapping from thecodeword offset mapping table according to the input digital codeword byan auxiliary DAC circuit to generate an offset calibration analogsignal, such that the offset calibration analog signal and the outputanalog signal are added to cancel the codeword offsets that the inputdigital codeword corresponds to output an actual output analog signal.9. The digital-to-analog conversion method in claim 7, wherein the inputdigital codeword comprises a plurality of thermometer codes to control aplurality of thermometer-control current sources of the current sources,the digital-to-analog conversion method further comprises: categorizingthe offset amount corresponding to the different input digital codewordinto one of the groups that corresponds to a combination of thethermometer codes and generating an average value of the offset amountof each of the groups by the calibration parameter calculating circuit,wherein the groups are arranged in an order such that for each two ofthe neighboring groups, the operation state of one of thethermometer-control current sources is at the first current output stateand the second current output state respectively; and generating thecurrent offset of one of the thermometer-control current sourcesaccording to a difference of the average value between each two of theneighboring groups by the calibration parameter calculating circuit. 10.The digital-to-analog conversion method in claim 9, wherein the inputdigital codeword further comprises a plurality of binary codes tocontrol a plurality of binary-control current sources of the currentsources, the digital-to-analog conversion method further comprises:selecting a first group and a second group from the groups by thecalibration parameter calculating circuit, wherein the thermometer codescorresponding to the first group and the second group make the operationstate of each of the thermometer-control current sources completelyopposite; and averaging the offset amount of each of the binary-controlcurrent sources that corresponds to the first current output state inthe first group and the second group by the calibration parametercalculating circuit to generate the current offset of one of thebinary-control current sources.
 11. The digital-to-analog conversionmethod in claim 10, wherein a number of the thermometer codes is A, thenumber of the thermometer-control current sources is 2^(A)-1, the numberof the groups is 2^(A), the number of the binary codes is B, the numberof the binary-control current sources is B, and the number of thecombination of the offset amount comprised in each of the groups is2^(B), and the number of the current offsets is 2^(A)-1+B.
 12. Thedigital-to-analog conversion method in claim 10, further comprising:setting the average value of each of the first group and the secondgroup to be 0, and generating the current offset of one of thethermometer-control current sources according to a difference betweenthe average values of each two of the neighboring groups by thecalibration parameter calculating circuit.